Running back Adrian Peterson limbers up before a preseason game against Denver. (John McDonnell/The Washington Post)
Sports columnist

Adrian Peterson’s assessment of himself after his first football game in nine months was plain: “I could have had another 15 carries.” His addition to the Washington Redskins’ roster early last week seemed like a lark, and then he gained 56 yards on 11 carries in an exhibition game. The reasonable among us said, “Maybe there’s something here.” The unhinged among us believed a 33-year-old who walked in off the street had ended Washington’s search for a bell cow back.

Let’s be realistic and center ourselves on a football truism: Running backs do not get better with age. They do not even maintain their level of play. They get worse. They just do.

This can be true at the same time that Peterson appears to be in fabulous shape, true at the same time Peterson ran so confidently behind Washington’s first line last week against Denver, true at the same time Coach Jay Gruden wants to get him more work in different circumstances — whether that’s in Thursday night’s exhibition finale in Baltimore or in practice or even in the Sept. 9 opener at Arizona.

Again, center yourself, and think rationally. Over the past 50 years, NFL running backs have produced 590 1,000-yard seasons. Here is the list of those seasons produced by backs 33 or older: Franco Harris in 1983, John Riggins in 1983 and 1984 and Frank Gore in 2016. That’s it.

Now, of course, Washington doesn’t need a 1,000-yard season from Peterson to make his inclusion in the backfield a success. Maybe 800 yards, just 50 a game? Riggins, Gore, Harris, Emmitt Smith, Larry Csonka and Marcus Allen — that’s the list of 33-or-older backs who have cranked out that many yards over the past half-century. Of them, only Allen averaged four yards a carry.

By pedigree, Peterson would fit into that group if he never had another NFL carry. Of those backs, only Gore, in camp with the Dolphins, isn’t in the Hall of Fame. He will get there after retirement. Peterson gained 2,097 yards in 2012 with Minnesota, the second-highest season total ever, and enters 2018 needing just 37 yards to move past Jim Brown and into the top 10 of career rushers. Canton can fit him for a golden blazer now.

But what can honestly be expected of Peterson here and now? Riggins was 34 when he gained 1,347 yards for Washington in 1983, 35 when he gained 1,239 yards the next season, the two most prolific years for a back age 33 or older.

During those seasons, Joe Gibbs, then the coach, gave Riggins the ball a stunning 702 times. No back of any age over the past two seasons has more than Le’Veon Bell’s 582 carries. The game has changed. Still, Riggins knows something about aging and workload.

“I also believe that there’s a little bit of age discrimination that goes on in the NFL by coaches because they just figure that it hasn’t been done or been done so infrequently,” Riggins told the crowd Wednesday at the Redskins’ annual Welcome Home Luncheon. “Okay, so what makes you think this guy’s still got it? But I think that he still has it. I saw everything.”

And at that point, the assembled crowd at the Marriott Marquis started applauding. Loudly.

Now, I hate to be the one to throw cold water on the Diesel and his acolytes . . . but where’s my bucket?

In 2016, the last of Peterson’s 10 years in Minnesota, he played in just three games because of a torn meniscus in his right knee. Following that year, the Vikings decided not to pick up his option, and he headed instead to New Orleans. There, he was ineffective (27 carries, 81 yards in four games), and the rise of rookie Alvin Kamara combined with the steadiness of veteran Mark Ingram made Peterson expendable.

But Peterson’s belief that he, as Riggins said, “still has it” comes from what came after a midseason trade to Arizona. His debut produced 134 yards on 26 carries. Two weeks later, he carried the ball a staggering 37 times and gained 159 yards.

That’s enough to convince a former MVP that he, um, still has it. It’s enough to make someone of Riggins’s stature look at those stats, consider what he saw last week in the exhibition against Denver and say he still has it.

But it’s also an outright anomaly. Guess how many times a back age 32 or older has carried the ball at least 37 times in a game over the past 50 years. Think about it.

Okay, time’s up. The answer: One (1), and it’s Adrian Peterson for Arizona at San Francisco on Nov. 5, 2017.

Sorry, John, that’s not age discrimination. That’s a practical matter. Older backs don’t get the workload, because their bodies can no longer handle the workload.

“The big question is,” Riggins continued at the luncheon, “can you do it 25 times a game for 16 games?”

Lindsay Czarniak, the announcer who interviewed Riggins onstage, looked at Peterson and said over the din, “Can you do it 25 times a game?” She waited for an answer before saying, “Of course, of course, of course!”

Tap the brakes. Please.

Look, there might be something here. While Peterson enters this season 12th in career rushing yards, he’s just 22nd in attempts, so maybe the banging and bruising hasn’t slowed him as much. He appears in otherworldly physical condition. Washington doesn’t need him to produce a 1,000-yard season or an 800-yard season, which is good, because those aren’t realistic. Washington has Chris Thompson and Samaje Perine and Rob Kelley to join Peterson in piecing some sort of running game together.

Plus, we all need to remember Peterson’s final three games with Arizona yielded a total of 134 yards, that he missed two games because of a neck injury, that he ended the year on injured reserve and was ultimately released and that he wouldn’t be in Ashburn — and wouldn’t be subject to Riggins’s assessment — had Washington rookie Derrius Guice not gone down with a season-ending knee injury in the first exhibition game.

Which is a roundabout way of saying the business of playing running back in the NFL is cruel, regardless of your age. Now 2,574 carries into his career, Adrian Peterson has to know that his next could be his last. The Redskins, therefore, must treat any positive yards he gives them in 2018 as a bonus, not an expectation.

For more by Barry Svrluga, visit washingtonpost.com/svrluga.